At fertilization the union of the sperm and egg has two important functions: it brings together their genetic components and it triggers embryonic development. It is oNen assumed that only the genetic component has an important part to play in further development. In the following review it is argued, on the basis of recent findings, that in disregarding the initial trigger for development, a rise in cytoplasmic Ca²⁺, an important factor in the control of embryonic development is being ignored. The review will concentrate on mammalian eggs due to the very characteristic Ca²⁺ changes which constitute the trigger for egg activation, and so embryonic development. Mammalian eggs show low.frequency Ca²⁺ oscillations over a period of several hours. It will be argued that a novel mechanism of Ca²⁺ release is employed by the sperm at fertilization and such Ca²⁺ changes have a role in subsequent development of the embryo in addition to the initial events of egg activation. For the purposes of this review the female gamete is defined as an 'egg' when it is at a stage that it is normally fertilized, in contrast to an 'oocyte' when it is at an immature stage of maturation and would not normally be fertilized. For all the species discussed here only in the sea urchin is the female gamete correctly termed an egg, having completed all the stages of meiosis.